Ecu & Frequency Valve
ECU continually corrects air/fuel mixture, based on signals from O2 sensor. ECU sends signals to frequency valve, which is located in fuel line between upper and lower halves of fuel distributor. Frequency valve operates at constant frequency (70 Hz) as soon as engine is running.
When frequency valve is closed, fuel pressure to injectors is determined by spring in each pressure regulating valve. When frequency valve is open, fuel pressure decreases in lower half of fuel distributor, tension on spring is relieved, and more fuel is directed to cylinders. See Fig 1 .
ECU grounds connection for frequency valve in different length pulses. This will open and close valve to ensure continuous regulation of fuel mixture. When engine is cold, ratio of valve open to valve closed is about 60/40.
After engine warms up, voltage produced by O2 sensor determines amount of time frequency valve must be open or closed. This ratio can be read with special tester or with dwell meter (on most models). Dwell reading of 45° indicates ratio of 50% open, 50% closed.